<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
    "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">
  <head>
    <meta name="generator" content="HTML Tidy, see www.w3.org" />

    <title>Connections in FIN_WAIT_2 and Apache</title>
  </head>
  <!-- Background white, links blue (unvisited), navy (visited), red (active) -->

  <body bgcolor="#FFFFFF" text="#000000" link="#0000FF"
  vlink="#000080" alink="#FF0000">
    <!--#include virtual="header.html" -->

    <h1 align="CENTER">Connections in the FIN_WAIT_2 state and
    Apache</h1>

    <ol>
      <li>
        <h2>What is the FIN_WAIT_2 state?</h2>
        Starting with the Apache 1.2 betas, people are reporting
        many more connections in the FIN_WAIT_2 state (as reported
        by <code>netstat</code>) than they saw using older
        versions. When the server closes a TCP connection, it sends
        a packet with the FIN bit sent to the client, which then
        responds with a packet with the ACK bit set. The client
        then sends a packet with the FIN bit set to the server,
        which responds with an ACK and the connection is closed.
        The state that the connection is in during the period
        between when the server gets the ACK from the client and
        the server gets the FIN from the client is known as
        FIN_WAIT_2. See the <a
        href="ftp://ds.internic.net/rfc/rfc793.txt">TCP RFC</a> for
        the technical details of the state transitions. 

        <p>The FIN_WAIT_2 state is somewhat unusual in that there
        is no timeout defined in the standard for it. This means
        that on many operating systems, a connection in the
        FIN_WAIT_2 state will stay around until the system is
        rebooted. If the system does not have a timeout and too
        many FIN_WAIT_2 connections build up, it can fill up the
        space allocated for storing information about the
        connections and crash the kernel. The connections in
        FIN_WAIT_2 do not tie up an httpd process.</p>
      </li>

      <li>
        <h2>But why does it happen?</h2>
        There are numerous reasons for it happening, some of them
        may not yet be fully clear. What is known follows. 

        <h3>Buggy clients and persistent connections</h3>
        Several clients have a bug which pops up when dealing with
        <a href="../keepalive.html">persistent connections</a> (aka
        keepalives). When the connection is idle and the server
        closes the connection (based on the <a
        href="../mod/core.html#keepalivetimeout">KeepAliveTimeout</a>),
        the client is programmed so that the client does not send
        back a FIN and ACK to the server. This means that the
        connection stays in the FIN_WAIT_2 state until one of the
        following happens: 

        <ul>
          <li>The client opens a new connection to the same or a
          different site, which causes it to fully close the older
          connection on that socket.</li>

          <li>The user exits the client, which on some (most?)
          clients causes the OS to fully shutdown the
          connection.</li>

          <li>The FIN_WAIT_2 times out, on servers that have a
          timeout for this state.</li>
        </ul>

        <p>If you are lucky, this means that the buggy client will
        fully close the connection and release the resources on
        your server. However, there are some cases where the socket
        is never fully closed, such as a dialup client
        disconnecting from their provider before closing the
        client. In addition, a client might sit idle for days
        without making another connection, and thus may hold its
        end of the socket open for days even though it has no
        further use for it. <strong>This is a bug in the browser or
        in its operating system's TCP implementation.</strong></p>

        <p>The clients on which this problem has been verified to
        exist:</p>

        <ul>
          <li>Mozilla/3.01 (X11; I; FreeBSD 2.1.5-RELEASE
          i386)</li>

          <li>Mozilla/2.02 (X11; I; FreeBSD 2.1.5-RELEASE
          i386)</li>

          <li>Mozilla/3.01Gold (X11; I; SunOS 5.5 sun4m)</li>

          <li>MSIE 3.01 on the Macintosh</li>

          <li>MSIE 3.01 on Windows 95</li>
        </ul>

        <p>This does not appear to be a problem on:</p>

        <ul>
          <li>Mozilla/3.01 (Win95; I)</li>
        </ul>

        <p>It is expected that many other clients have the same
        problem. What a client <strong>should do</strong> is
        periodically check its open socket(s) to see if they have
        been closed by the server, and close their side of the
        connection if the server has closed. This check need only
        occur once every few seconds, and may even be detected by a
        OS signal on some systems (<em>e.g.</em>, Win95 and NT
        clients have this capability, but they seem to be ignoring
        it).</p>

        <p>Apache <strong>cannot</strong> avoid these FIN_WAIT_2
        states unless it disables persistent connections for the
        buggy clients, just like we recommend doing for Navigator
        2.x clients due to other bugs. However, non-persistent
        connections increase the total number of connections needed
        per client and slow retrieval of an image-laden web page.
        Since non-persistent connections have their own resource
        consumptions and a short waiting period after each closure,
        a busy server may need persistence in order to best serve
        its clients.</p>

        <p>As far as we know, the client-caused FIN_WAIT_2 problem
        is present for all servers that support persistent
        connections, including Apache 1.1.x and 1.2.</p>

        <h3>A necessary bit of code introduced in 1.2</h3>
        While the above bug is a problem, it is not the whole
        problem. Some users have observed no FIN_WAIT_2 problems
        with Apache 1.1.x, but with 1.2b enough connections build
        up in the FIN_WAIT_2 state to crash their server. The most
        likely source for additional FIN_WAIT_2 states is a
        function called <code>lingering_close()</code> which was
        added between 1.1 and 1.2. This function is necessary for
        the proper handling of persistent connections and any
        request which includes content in the message body
        (<em>e.g.</em>, PUTs and POSTs). What it does is read any
        data sent by the client for a certain time after the server
        closes the connection. The exact reasons for doing this are
        somewhat complicated, but involve what happens if the
        client is making a request at the same time the server
        sends a response and closes the connection. Without
        lingering, the client might be forced to reset its TCP
        input buffer before it has a chance to read the server's
        response, and thus understand why the connection has
        closed. See the <a href="#appendix">appendix</a> for more
        details. 

        <p>The code in <code>lingering_close()</code> appears to
        cause problems for a number of factors, including the
        change in traffic patterns that it causes. The code has
        been thoroughly reviewed and we are not aware of any bugs
        in it. It is possible that there is some problem in the BSD
        TCP stack, aside from the lack of a timeout for the
        FIN_WAIT_2 state, exposed by the
        <code>lingering_close</code> code that causes the observed
        problems.</p>
      </li>

      <li>
        What can I do about it? There are several possible
        workarounds to the problem, some of which work better than
        others. 

        <h3>Add a timeout for FIN_WAIT_2</h3>
        The obvious workaround is to simply have a timeout for the
        FIN_WAIT_2 state. This is not specified by the RFC, and
        could be claimed to be a violation of the RFC, but it is
        widely recognized as being necessary. The following systems
        are known to have a timeout: 

        <ul>
          <li><a href="http://www.freebsd.org/">FreeBSD</a>
          versions starting at 2.0 or possibly earlier.</li>

          <li><a href="http://www.netbsd.org/">NetBSD</a> version
          1.2(?)</li>

          <li><a href="http://www.openbsd.org/">OpenBSD</a> all
          versions(?)</li>

          <li><a href="http://www.bsdi.com/">BSD/OS</a> 2.1, with
          the <a
          href="ftp://ftp.bsdi.com/bsdi/patches/patches-2.1/K210-027">
          K210-027</a> patch installed.</li>

          <li><a href="http://www.sun.com/">Solaris</a> as of
          around version 2.2. The timeout can be tuned by using
          <code>ndd</code> to modify
          <code>tcp_fin_wait_2_flush_interval</code>, but the
          default should be appropriate for most servers and
          improper tuning can have negative impacts.</li>

          <li><a href="http://www.linux.org/">Linux</a> 2.0.x and
          earlier(?)</li>

          <li><a href="http://www.hp.com/">HP-UX</a> 10.x defaults
          to terminating connections in the FIN_WAIT_2 state after
          the normal keepalive timeouts. This does not refer to the
          persistent connection or HTTP keepalive timeouts, but the
          <code>SO_LINGER</code> socket option which is enabled by
          Apache. This parameter can be adjusted by using
          <code>nettune</code> to modify parameters such as
          <code>tcp_keepstart</code> and <code>tcp_keepstop</code>.
          In later revisions, there is an explicit timer for
          connections in FIN_WAIT_2 that can be modified; contact
          HP support for details.</li>

          <li><a href="http://www.sgi.com/">SGI IRIX</a> can be
          patched to support a timeout. For IRIX 5.3, 6.2, and 6.3,
          use patches 1654, 1703 and 1778 respectively. If you have
          trouble locating these patches, please contact your SGI
          support channel for help.</li>

          <li><a href="http://www.ncr.com/">NCR's MP RAS Unix</a>
          2.xx and 3.xx both have FIN_WAIT_2 timeouts. In 2.xx it
          is non-tunable at 600 seconds, while in 3.xx it defaults
          to 600 seconds and is calculated based on the tunable
          "max keep alive probes" (default of 8) multiplied by the
          "keep alive interval" (default 75 seconds).</li>

          <li><a href="http://www.sequent.com">Sequent's ptx/TCP/IP
          for DYNIX/ptx</a> has had a FIN_WAIT_2 timeout since
          around release 4.1 in mid-1994.</li>
        </ul>

        <p>The following systems are known to not have a
        timeout:</p>

        <ul>
          <li><a href="http://www.sun.com/">SunOS 4.x</a> does not
          and almost certainly never will have one because it as at
          the very end of its development cycle for Sun. If you
          have kernel source should be easy to patch.</li>
        </ul>

        <p>There is a <a
        href="http://www.apache.org/dist/httpd/contrib/patches/1.2/fin_wait_2.patch">
        patch available</a> for adding a timeout to the FIN_WAIT_2
        state; it was originally intended for BSD/OS, but should be
        adaptable to most systems using BSD networking code. You
        need kernel source code to be able to use it. 

        <h3>Compile without using
        <code>lingering_close()</code></h3>
        It is possible to compile Apache 1.2 without using the
        <code>lingering_close()</code> function. This will result
        in that section of code being similar to that which was in
        1.1. If you do this, be aware that it can cause problems
        with PUTs, POSTs and persistent connections, especially if
        the client uses pipelining. That said, it is no worse than
        on 1.1, and we understand that keeping your server running
        is quite important. 

        <p>To compile without the <code>lingering_close()</code>
        function, add <code>-DNO_LINGCLOSE</code> to the end of the
        <code>EXTRA_CFLAGS</code> line in your
        <code>Configuration</code> file, rerun
        <code>Configure</code> and rebuild the server.</p>

        <h3>Use <code>SO_LINGER</code> as an alternative to
        <code>lingering_close()</code></h3>
        On most systems, there is an option called
        <code>SO_LINGER</code> that can be set with
        <code>setsockopt(2)</code>. It does something very similar
        to <code>lingering_close()</code>, except that it is broken
        on many systems so that it causes far more problems than
        <code>lingering_close</code>. On some systems, it could
        possibly work better so it may be worth a try if you have
        no other alternatives. 

        <p>To try it, add <code>-DUSE_SO_LINGER
        -DNO_LINGCLOSE</code> to the end of the
        <code>EXTRA_CFLAGS</code> line in your
        <code>Configuration</code> file, rerun
        <code>Configure</code> and rebuild the server.</p>

        <p><strong>NOTE:</strong> Attempting to use
        <code>SO_LINGER</code> and <code>lingering_close()</code>
        at the same time is very likely to do very bad things, so
        don't.</p>

        <h3>Increase the amount of memory used for storing
        connection state</h3>

        <dl>
          <dt>BSD based networking code:</dt>

          <dd>
            BSD stores network data, such as connection states, in
            something called an mbuf. When you get so many
            connections that the kernel does not have enough mbufs
            to put them all in, your kernel will likely crash. You
            can reduce the effects of the problem by increasing the
            number of mbufs that are available; this will not
            prevent the problem, it will just make the server go
            longer before crashing. 

            <p>The exact way to increase them may depend on your
            OS; look for some reference to the number of "mbufs" or
            "mbuf clusters". On many systems, this can be done by
            adding the line <code>NMBCLUSTERS="n"</code>, where
            <code>n</code> is the number of mbuf clusters you want
            to your kernel config file and rebuilding your
            kernel.</p>
          </dd>
        </dl>

        <h3>Disable KeepAlive</h3>

        <p>If you are unable to do any of the above then you
        should, as a last resort, disable KeepAlive. Edit your
        httpd.conf and change "KeepAlive On" to "KeepAlive
        Off".</p>
      </li>


      <li>
      <h2><a id="appendix" name="appendix">Appendix</a></h2>

        <p>Below is a message from Roy Fielding, one of the authors
        of HTTP/1.1.</p>

        <h3>Why the lingering close functionality is necessary with
        HTTP</h3>
        The need for a server to linger on a socket after a close
        is noted a couple times in the HTTP specs, but not
        explained. This explanation is based on discussions between
        myself, Henrik Frystyk, Robert S. Thau, Dave Raggett, and
        John C. Mallery in the hallways of MIT while I was at W3C. 

        <p>If a server closes the input side of the connection
        while the client is sending data (or is planning to send
        data), then the server's TCP stack will signal an RST
        (reset) back to the client. Upon receipt of the RST, the
        client will flush its own incoming TCP buffer back to the
        un-ACKed packet indicated by the RST packet argument. If
        the server has sent a message, usually an error response,
        to the client just before the close, and the client
        receives the RST packet before its application code has
        read the error message from its incoming TCP buffer and
        before the server has received the ACK sent by the client
        upon receipt of that buffer, then the RST will flush the
        error message before the client application has a chance to
        see it. The result is that the client is left thinking that
        the connection failed for no apparent reason.</p>

        <p>There are two conditions under which this is likely to
        occur:</p>

        <ol>
          <li>sending POST or PUT data without proper
          authorization</li>

          <li>sending multiple requests before each response
          (pipelining) and one of the middle requests resulting in
          an error or other break-the-connection result.</li>
        </ol>

        <p>The solution in all cases is to send the response, close
        only the write half of the connection (what shutdown is
        supposed to do), and continue reading on the socket until
        it is either closed by the client (signifying it has
        finally read the response) or a timeout occurs. That is
        what the kernel is supposed to do if SO_LINGER is set.
        Unfortunately, SO_LINGER has no effect on some systems; on
        some other systems, it does not have its own timeout and
        thus the TCP memory segments just pile-up until the next
        reboot (planned or not).</p>

        <p>Please note that simply removing the linger code will
        not solve the problem -- it only moves it to a different
        and much harder one to detect.</p>
      </li>
    </ol>
    <!--#include virtual="footer.html" -->
  </body>
</html>

